Glass beads and microspheres are known in the art and have been developed for and used in a variety of applications. Glass beads have been used in retroreflective products, as fillers, in propping and peening applications, and in optical devices. Compositions of these known beads have generally been limited to traditional glass-forming compositions, or to high refractive index compositions with favorable melting and processing behavior. For example, pavement marking beads of soda-lime-silica glass comprising about 70 percent silica are common. High refractive index beads typically comprise less silica and have substantial amounts of titania, baria, lead, or bismuth. High index pavement marking beads have been doped with rare earth elements to provide visibility-enhancing fluorescence. Beads for mechanical uses often have significant amounts of alumina or zirconia.
Beads used in optical devices have been derived from high purity optical materials such as optical fibers and laser glass. Such materials have provided the desired ultra high-Q factors, or low losses, desired for resonator devices. Accordingly, beads of this type have included pure silica beads, pure fluoride glass beads (so-called ZBLA and ZBLAN), and beads of high phosphate laser glass. For resonator applications, these glasses are sometimes doped with a low level of a rare earth agent to make them optically active. Beads made by melting the tip of an optical fiber comprise primarily pure silica or pure fluoride glass, but have a very small center region comprising additional components derived from the core region of the optical fiber.
In addition, high purity glass is essential for applications in which the glass transmits light, as in a waveguide, optical fiber, or in high-Q resonators. It is known that transition metal or rare earth impurities can strongly absorb visible and infrared light, which leads to increased optical loss in a device. For example, transition metals, such as iron, copper, and vanadium, have crystal field splitting energies in the 1-10 eV range (˜1240-˜125 nm) and broad absorption bands, which are deleterious in the visible and near-IR regions. The presence of iron (III) in silica, for example, can lead to an induced absorption of 130 dB/km/ppm at 1.3 μm. Similarly, rare earth ions exhibit strong, but narrow, absorption bands in the visible and IR spectra. For example, Tb3+ in fluorozirconate induces a 150 dB/km/ppm absorption at 3.0 μm in fluorozirconate.
In addition, impurity ions can alter the local structure of a glass and lead to different crystal field environments around nearby cations. In the case of rare-earth-doped glasses, the local field dictates the lifetime and breadth of the emission spectrum. As the use of high purity glass is essential in transmission applications, for example, amplifier optical fiber, it is prudent to use high purity glasses to screen compositions in order get the most accurate information about how that glass will perform in an optical device.
In silicates, hydroxyl ions impart unwanted absorption bands at 2.75, 2.22, 1.38, 1.24, and 0.95 μm. The 1.38 μm absorption band is particularly problematic for telecommunications applications. In silicate optical fiber, the absorption results in about a 40 dB/km/ppm hydroxyl ion loss at 1.38 μm. Thus, it is desirable for telecommunications devices and waveguides to have hydroxyl concentrations less than about 1 ppm. The presence of hydroxyl ions has also been reported to decrease the excited-state lifetime of rare-earth-doped glasses. Hydroxyl ions also modify the viscosity of the glass. The log viscosity decreases about 0.0018/ppm hydroxyl ion. For example, 100 ppm hydroxyl ions in the glass decreases the viscosity of the glass by approximately 40 percent.
What is needed are homogeneous beads, processes of making, and devices comprising high purity, tailored compositions that provide non-maximal Q-factors. Such beads would be useful for broadening selected frequency bands in resonators, for photo-trimmable devices, and for screening glass compositions for making optical devices, for example, optical fibers.